The present invention relates to processes for forming structures by a transferring of a thin layer of semiconducting material from a donor substrate to a support substrate. One field of application is in the field of semiconductor-on-insulator (SeOI) structures, for example, silicon-on-insulator (SOI) structures, that provide structures acting as substrates for electronics, optics and optoelectronics. SeOI structures are formed to include an insulating layer inserted between a thin layer made of a semiconducting material and a support substrate. SMART CUT® type processes are an example of a process of this type. These processes correspond to at least some embodiments of the present invention.
Production of an SeOI structure using the SMART CUT® process, and particularly an SeOI structure in which the thin layer is particularly thin (typically less than 400 nm), requires the use of an initial donor substrate without any growth defects in the form of vacancy clusters (known, for instance, as Crystal Originated Particles or COPs). The presence of vacancy clusters in the volume of the donor substrate may generate defects for which the size is larger than the thickness of the thin layer of the final SeOI structure. These resulting “through” defects are fatal defects because a component formed in a portion of the SeOI structure that includes one of these defects will not be operable. The presence of these through defects is, therefore, a parameter that controls the quality of components that will be created on the final structure. Therefore, it is essential to minimize the presence of these through defects. Obviously, it will be understood that the problem of such defects is particularly important if the thickness of the thin layer is “thin,” such that the size of vacancy clusters are significant in comparison to the thickness of the layer.
One solution for limiting the number of through defects in an SeOI substrate and that has been frequently used in the past is to use an initial substrate with a very high crystallographic quality and having a low density of COPs.
An initial substrate is typically formed by cutting into an ingot obtained by the CZ process (Czochralski pulling). Control over the pulling speed and the ingot-cooling rate provides a means for reducing the quantity of vacancy cluster type defects. Thus, an initial substrate with almost no COPs is typically formed by cutting an ingot obtained by the CZ (Czochralski pulling) process using very specific pulling conditions and, in particular, using very slow pulling (pulling also designated by the name “Very Slow Pull” to obtain what those skilled in the art call a “Near Perfect Crystal,” due to the very low number of defects).
Substrates formed by cutting an ingot obtained by simpler and/or faster pulling processes have comparatively more vacancy clusters and are, therefore, considered to be incompatible with the constraints imposed in target application fields (such as optics, electronics or optoelectronics).
For example, a substrate formed by cutting a Near Perfect Crystal obtained by a “Very Slow Pull” type pulling at a rate of less than 0.5 mm/min. will typically have a density of COPs (larger than 0.1 μm) between 0.045 and 0.075 COPs/cm2 (equivalent to 30 to 50 COPs larger than 0.1 μm in a 300 mm diameter wafer with a surface area of 660 cm2, allowing for a 5 mm exclusion area around the wafer). In comparison, a substrate obtained using standard pulling at a rate 1.2 to 1.5 times faster than the “Very Slow Pull” type pulling, will have a density of COPs (larger than 0.1 μm) between 1.5 and 4.5 COPs/cm2 (equivalent to 1000 to 3000 COPs larger than 0.1 μm in a 300 mm diameter wafer).
It will be noted that the ingot-cooling rate during pulling is another factor that will influence the crystallographic quality. Indeed a high cooling rate (pulling designated as “Fast Cool”) is accompanied by an increase in the density of defects. Therefore, a substrate obtained by cutting an ingot obtained by “Fast Cool” type pulling is also incompatible with imposed constraints in the application fields of the present invention.
The production efficiency for a quality substrate (Near Perfect Crystal with almost no COPs by means of a CZ “Very Slow Pull” type pulling) is significantly lower than the production efficiency of substrates using simpler and/or faster pulling processes. Therefore, the production of a near perfect substrate by “Very Slow Pull” type pulling is particularly expensive; its cost is thus typically 30% greater than the cost of substrates obtained by standard CZ pulling.
It will be noted that it has also been proposed to use a previously heat-treated standard substrate to reduce the quantity of COPs as an initial substrate in a process for making an SOI structure. However, the use of such a previous heat treatment is not satisfactory. This treatment modifies the surface properties of the initial substrate (and, in particular, increases its surface roughness) such that problems can occur during bonding of the initial substrate to a support substrate (and particularly degradation of the bonding quality). Furthermore, this previous heat treatment can generate “slip line” type defects or oxygen precipitates that could compromise recycling of the initial substrate that is usually used in a SMART CUT® type process.
In order to improve the vacancy cluster characteristics of semiconductor structures, the assignee has proposed in published application US 2006/0172508 methods for forming a semiconductor-on-insulator structure, in which a donor substrate having a first density of vacancy clusters is provided, and wherein, a thin layer is transferred from the donor substrate to the support substrate and the transferred thin layer is cured to reduce the first density of vacancy clusters to a second density of vacancy clusters.
In particular, curing of the transferred thin layer can include applying a thermal annealing to the structure.
The methods of US 2006/0172508 notably allow reducing the price cost of substrates made by transfer of a thin layer of semiconducting material from a donor substrate to a support substrate and, particularly, the price cost of SeOI substrates. More specifically, the methods of US 2006/0172508 enable the use of initial substrates formed by cutting an ingot obtained by CZ pulling, which is more cost effective than the “Very Slow Pull” type pulling to obtain a near perfect crystal, in a SMART CUT® type transfer process, while maintaining a crystalline quality of the thin layer compatible with the envisaged applications, without encountering any bonding or recycling type problems caused by use of the previous heat treatment technique mentioned above.
The inventors of the present application have, however, shown that the curing step, typically a thermal annealing, of the methods of US 2006/0172508 can have certain limitations.
Typically, the vacancy clusters are cavities, with sizes ranging between a few hundreds and a few thousands of Angstroms, which inner walls are coated by a thin silicon oxide film. This coating must be dissolved before the thermal annealing of the curing step can be effective.
It appeared that the thermal annealing of the curing step has a side effect linked to the thermal balance of the oxygen concentration in the depth of the thin layer. Indeed, the thermal annealing may cause oxygen, in particular, oxygen present within the insulating buried layer (for instance, a silicon oxide layer), to diffuse toward the thin layer and migrate to the inner walls of the vacancy clusters.
In addition to a possible thinning of the thin layer, this diffusion of oxygen, in particular, oxygen of the buried layer, toward the free face of the thin layer may reduce the effectiveness of the dissolution of the vacancy clusters by the methods of US 2006/0172508.
Therefore, it would be beneficial to provide improved methods with more effective vacancy clusters dissolution, at least, for example, for forming SeOI structures having improved defect characteristics and processing efficiencies.